Is this a transistor :
There are 2 PCB's visible in your pictures.The second PCB, which is only visible in the 3rd. picture might have a relay (relais) on the invisible side.You can probably alter the PCB so that that relay will not be connected to the 230 volts DC anymore.Then you can use those contacts to switch your Arduino.This still will require you to power the PIR sensor from 230 V, and i'm hesitating to advise you to do so (because you have to ask this question).
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via Egeo 16